Polyethylene is the most widely used commercial polymer. It can be prepared by a couple of different processes with a broad range of achievable properties. There is however still the wish to improve the products. One measure to enhance the property balance of polyethylenes is preparing so-called bimodal or multimodal polyethylenes. Bimodal polyethylenes are usually obtained by polymerizing or copolymerizing ethylene and optionally comonomers at two different polymerization conditions, which can either arise from polymerizing in a two stage cascade polymerization process at different polymerization conditions or from using mixed or hybrid catalysts, which have different kinds of active sites on one catalyst particle (F. Alt et al., Macromol. Symp. 2001, 163, 135-143). Such polyethylenes have usually a bimodal or at least broad molecular weight distribution and have often also an optimized comonomer distribution. If using a cascade of more than two stages or a mixed catalyst with more than two different kinds of active sites the obtained polymers are no longer “bimodal”. It is therefore common to utilize for such situations the term “multimodal”, while however the term “multimodal” is often also used in the sense of “more than one mode” and consequently including “bimodal”. In the present description the term “multimodal” is as well utilized with the latter meaning, i.e. including “bimodal”.
The advantages of the approach using a mixed catalyst system for preparing multimodal polyethylenes are that it is sufficient to employ only one polymerization reactor and that the produced polyethylenes have a better homogeneity, and thus improved properties, than those resulting from a cascade process since all catalyst particles of a mixed catalyst system are polymerized at the same polymerization conditions, while in a cascade process different catalyst particles have different residence times at the different polymerization conditions and therefore different polymer compositions. However, on the other hand, in a polymerization process with a mixed catalyst system there are only limited possibilities to control the resulting polymer composition because all active sites of the mixed catalyst system undergo the same variations of the polymerization conditions.
WO 96/09328 describes an ethylene polymerization process with a mixed catalyst system comprising a metallocene catalyst component and a Ziegler catalyst component in which water and/or carbon dioxide are co-fed to the polymerization reactor to control the molecular weight distribution of the obtained polyethylene. Also WO 2007/012406 discloses a method for controlling the relative activity of different active centers of a mixed catalyst system by polymerization in the presence of water and/or carbon dioxide, in which the catalyst system comprises a late transition metal catalyst component and a catalyst component comprising cyclopentadienyl ligands.
Another type of mixed catalyst systems for olefin polymerization is described in WO 2008/125208, which teaches mixed catalyst systems comprising a Ziegler catalyst component and a late transition catalyst component for producing multimodal polyethylenes which have good mechanical properties and good processability. WO 2009/080359 discloses that such catalyst systems may be selectively controlled by varying the amount of activating compound, thus allowing controlling the molecular weight and the comonomer composition of the obtained polyethylene fractions. By utilizing an activating compound for controlling the catalyst system it is however only possible to influence the properties of the polymers produced by the Ziegler catalyst component of the mixed catalyst system. WO 2012/084774 describes that by varying the polymerization temperature it is also possible to alter the polymer composition of an ethylene copolymer obtained by such a catalyst system. However, varying the concentration of activating compound or varying the polymerization temperature may also bring about further changes in the polymerization behavior and furthermore, using these interactions for controlling the polymer composition removes them from the options for modifying the polymerization conditions and limits over-all the possibilities for influencing the polymerization.
The use of saturated halogenated hydrocarbons to enhance the activity of Ziegler catalysts is e.g. described in WO 2003/010211. Nothing is however reported about the effect of these reagents on mixed catalyst systems comprising a Ziegler catalyst component and a late transition catalyst component.
Thus, it was the object of the present invention to overcome the disadvantages of the prior art and find independent measures for controlling the resulting polymer composition of a multimodal ethylene copolymer when preparing it with a mixed catalyst system comprising a Ziegler catalyst component and a late transition catalyst component. This should be possible without drastically changing the catalysts productivity under stable operating conditions and so give more variation possibilities and flexibility for adjusting the polymerization process and provide a method for producing different ethylene copolymer compositions with one mixed catalyst system.